Vertebrate cone photoreceptors are responsible for color and daytime vision. They have been extensively studied and much is known about their structure and function. For example, anatomical studies have described the morphologies of cone cells, the types of synpases they form and how they can adapt their shape in response to environmental and circadian factors. Furthermore, electrophysiological studies have described the physiological properties of cone responses to brief flashes of light and to steady illumination. Nevertheless there are many unanswered questions about cone structure and function. For example, although the physiological description of cone adaptation to illumination is well documented, the biochemical mechanism underlying this response is unknown. In addition, the mechanisms by which ribbon synapses form and are regulated are unknown. Finally many factors that influence cone photoreceptor shape and viability have yet to be discovered. In this proposal the investigators use zebrafish to identify genes essential for normal cone structure and function. The proposal relies heavily on the previous success of the investigator to identify genes essential for vertebrate cone function using a similar screening strategy. The specific aims of the proposal are: Specific Aim#l: Identify recessive mutations that are essential for normal cone photoreceptor function using an OKR behavioral screen followed by ERG analysis. Specific Aim#2: Perform histological analyses of the retinas of mutants with defects in cone photoreceptor function. Specific Aim #3: Identify the mutated genes by positional cloning and candidate gene analysis. All resources generated from this proposal will be made available to the scientific community. A specific plan for distributing the resources generated from this proposal has been designed with investigators at the zebrafish resource center in Oregon. This plan is presented as part of the proposal.